1. Field of Invention
The present invention relates to a motor controller, in particular a motor controller generating a pulse width modulation (PWM) signal with a spread spectrum to reduce the generated electromagnetic interference (EMI).
2. Description of Related Art
Referring to FIG. 1, a prior art motor controller 10 is shown. The motor controller 10 includes a compensator 11, a pulse width modulation unit 12, and a motor driving unit 13. The compensator 11 receives a reference signal Sref and a sensing signal from an external direct current motor, and generates a control signal Vc which is sent to the pulse width modulation unit 12; the motor driving unit 13 controls the external direct current motor according to the PWM signal generated by the pulse width modulation unit 12. The high frequency band of the PWM signal generated by the pulse width modulation unit 12 could cause EMI, and the direct current motor controlled by the PWM signal could be another source for generating EMI; for example, the operation of the direct current motor in a mobile phone for adjusting a focus of a lens could cause EMI to interfere signal reception. Conventional solutions to this problem include adding a shielding housing to isolate the electromagnetic wave, or adding an anti-EMI component, etc. These solutions might be effective but they inevitably increases the cost and consumes more materials. Another prior art solution is to reduce portion of the signal in the high frequency band as it is transformed by Fourier transformation; this solution might improve the EMI, but it changes the waveform and thus causes efficiency or precision problem in certain operational ranges.
Therefore, a solution for reducing the EMI, which does not require any extra component except the circuit itself and does not affect the overall efficiency and precision, is highly desired.